1. Field of the Invention
The present invention relates to a driving system for a semiconductor light emitting device. In particular, the present invention relates to improvements in performance of a laser exposing system to be used in a laser printer for printing images using distribution of static electric potentials formed by exposing laser beams emitted from a semiconductor laser as well as improvements in high quality images produced by the laser printer using the laser exposing system.
2. Description of the Related Art
Laser printers perform printing in the following manner: Pulsed laser beams are generated according to a pattern of an image to be printed; a charged photoconductive film is scanned with the laser beams for forming a static electricity image; the static electricity image controls toner adhesion thereto for forming the pattern to be printed: and the pattern is transferred on a surface of a printing stock. In the case of driving the semiconductor laser serving as a light source by the use of a pulsed current, a virtual junction temperature of the semiconductor laser rises with a time constant of a several microseconds to a several tens of microseconds after the application of current to the semiconductor laser. Generally, light output/current characteristics of the semiconductor laser are such that a threshold current 1 of laser oscillation is increased with the rise in virtual junction temperature. A conceptual diagram of the light output/current characteristics (2, 3) in the case where the virtual junction temperature changes from T to (T+ΔT) is shown in FIG. 1. A constant current source is usually used for driving the semiconductor laser because of its easy light intensity control; however, in the case of supplying a current of I0 using the constant current source, the light output of the semiconductor laser is reduced from the initial output 4 to the output with the raised temperature 5 due to the change in light output/current characteristics. Accordingly, waveforms (6, 7) of an electric current and an optical pulse when the constant pulsed current is applied are as shown in FIG. 2, and the light intensity changes with time although the electric current value is kept constant.
Such phenomenon is the so-called droop phenomenon, which causes occurrence of irregularity of a printed image. Further, an associated phenomenon is a thermal crosstalk, which is problematic in a laser array having a plurality of light emitting devices on one and the same chip. The thermal crosstalk is caused by the reduction in light output due to the virtual junction temperature rise like the droop phenomenon; however, the phenomenon is different from the droop phenomenon in that the virtual junction temperature rises not because of the conduction of the device emitting light but because of a conduction of an adjacent device. That is to say even when a driving current 8 for the device emitting light does not change, a temperature of the device rises due to heating caused by a fluctuation in driving current of the adjacent device to reduce a light output 10 of the device emitting light, resulting in generation of irregularity of the printed image.
Electronics Letters; Vol. 28; P. 1460; 1991 describes the following: in the constant voltage driving wherein the driving source supplies pulses of a constant voltage to cause the semiconductor laser to emit light, the driving current is increased because a rising voltage of the current/voltage characteristics of the semiconductor laser is reduced when the virtual junction temperature rises. This compensates for the reduction in light output to mitigate the droop phenomenon. That is to say, owing to the difference between the voltage/current characteristics (11, 12) due to the virtual junction temperature shown in FIG. 4, the current flowing through the semiconductor laser at the time of application of the constant voltage is increased from the initial current value 13 to the current value after temperature rise 14 to compensate for the reduction in light output.
Owing to a series resistance of the semiconductor laser, the current increment is not sufficient, and the effect of stabilizing the light output achieved by the above method is so limitative that the light output moves to the point 15 of FIG. 1. Therefore, it has been necessary to employ this method in combination with another droop reduction method such as biasing with a threshold electric current at the time of non-energization.
Japanese Patent Laid-open No. 5-129899 discloses a negative resistance circuit proposed for the realization of a bistable circuit. The object of the negative resistance circuit is different from that of the present invention which aims to achieve a stable driving of semiconductor lasers. In addition, the method of forming the negative resistance circuit is similar to that of a circuit described in embodiments of this invention.
Japanese Patent Laid-open No. 7-297448 discloses a negative resistance device arranged in series with a light emitting device so as to control on/off of the light emitting device. In spite of the structure of the serial provision of the negative resistance device and the light emitting device, the object thereof is bistability, and the structure does not have a function of compensating for a fluctuation in output light at a stable point, which is described in the present specification.
According to Japanese Patent Laid-open No. 05-13850, a bias current is changed depending on a change in threshold current caused by a temperature fluctuation in a driving circuit of an optical communication semiconductor laser, whereby the bias current whose value is substantially the same as that of the threshold current is continuously applied. Since a temperature change in gate voltage of an FET is used for controlling the bias current, it is possible to arrange the structure so as to equalize a change in bias current with a value corresponding to the temperature change in threshold current. The temperature change in this document corresponds to a change in ambient temperature common to both the FET and the semiconductor laser; however, the it does not cope with the fluctuation due to the virtual junction temperature change in laser output which is more local and rapid in response speed unlike the present invention which takes advantage of the temperature change in voltage of the semiconductor laser itself.
Electronics Letters: Vol. 28; P. 1460; 1991 describes the following; When a semiconductor laser for a laser printer is driven for light emission by a constant voltage circuit for generating a constant pulsed voltage, a current flowing through the semiconductor laser is increased with an increase in virtual junction temperature to compensate for a reduction in light output due to the virtual junction temperature rise, thereby reducing fluctuation in light output due to the temperature fluctuation.
The present invention provides a simple circuit structure capable of preventing a light output fluctuation which is otherwise caused by a temperature change due to energization of a semiconductor laser under the driving condition of small bias current.